1. Field of the Invention
The present invention relates to a wireless communication system, a pilot sequence allocation apparatus and a method for allocating pilot sequence used for the system and the apparatus and a mobile station used in the method, and more specifically to the allocation of pilot sequence in a single carrier transmission system used in a wireless accessing method.
2. Description of the Prior Art
As an uplink wireless accessing method in the wireless communication system of the next generation, a single carrier transmission method is effective (refer to, for example, a Non-Patent document 1, “Physical Layer Aspects for Evolved UTRA” (3GPP TR25.814 v1.2.2 (2006-3), Chapter 9.1.). The configuration of a frame format used in the singe carrier transmission method proposed in the Non-Patent document 1 is shown in FIG. 19.
In FIG. 19, a data signal is supposed to be sent in six LB (Long Block) #1 to #6 in a sub frame and a pilot signal is supposed to be sent in two SB (Short Block) #1, #2.
CP (Cyclic Prefix) is added to the first half of the LB #1 to #6 and SB #1, #2 for effectively executing equalization of frequency regions at the receiving side. Addition of CP is to copy the latter portion of the block to the first portion as shown in FIG. 20.
As a pilot signal used in an uplink wireless accessing in the mobile communication system of the next generation, the Zadoff-Chu sequence, which is one of CAZAC (Constant Amplitude Zero Auto-Correlation) sequences, (refer to, for example, a Non-Patent document 2, K. Fazel and S. Keiser, “Multi-Carrier and Spread Spectrum Systems” (John Willey and Sons, 2003)) is currently drawing attention.
The Zadoff-Chu sequence is represented by the formula:C_k(n)=exp[−(j2πk/N)n(n+1)/2+qn)]  (1).In the formula (1), n=0, 1, . . . , N, and q is an arbitrary integer and N is a sequence length.
The CAZAC sequence is a sequence that has constant amplitude in both regions of time and frequency and it has always Zero Auto-Correlation for time shift that is other than the cyclic self-correlation value is 0. As the CAZAC sequence has Constant amplitude in a time region, it can keep PAPR (Peak to Average Power Ratio) low. As the CAZAC sequence also has Constant amplitude in a frequency region, it is a sequence suitable for propagation path estimation in the frequency region. Here, a small PAPR means that it can keep the power consumption low. This feature is preferred in the mobile communication.
Further, as the “CAZAC sequence” has a complete self-correlating characteristic, it is advantageous in being appropriate for detecting a time of a received signal and draws attention as the pilot sequence appropriate for a single carrier transmission, which is an uplink wireless accessing method in the wireless communication system of the next generation.
In the cellular environment (wireless communication network with a service area divided into a plurality of cells), the base station receives not only an uplink signal of the mobile station in the cell managed by the base station as an uplink received signal but also an uplink signal of the mobile station of the other cell (particularly, adjacent cell) (see FIG. 1). The mobile station receives not only a downlink signal from the base station of the cell managed by the base station but also a downlink signal of the base station of the other cell as it receives the uplink signal. Here, communication from the mobile station to the base station is called an uplink and communication from the base station to the mobile station is called a downlink. The above-mentioned cell can also be called a sector.
As the base station captures a pilot signal from the mobile station in the cell managed by the base station in the uplink communication, the pilot signal sent from the mobile station of the other cell needs to be reduced sufficiently. Thus, it is desirable that a set of small sequences of correlation values is allocated as the pilot sequence of cells adjacent to each other. In the downlink communication, it is also desirable that a set of small sequences of correlation values is allocated as the pilot sequence of cells adjacent to each other for the same reason for the uplink communication.
The correlation characteristic of the CAZAC sequence largely depends on the sequence length. That is to say, if the sequence length includes a prime number or a big prime number, the correlate characteristic is very good (correlation value is small). In contrast, if the sequence length is a combined number comprising only small prime numbers (for example, an exponent such as 2 or 3), the correlate characteristic greatly degrades (a big value is included in the correlation value).
Specifically, if the sequence length of Zadoff-Chu sequence is a prime number, the correlation value of arbitrary sequences is always kept 1/√{square root over ( )}N (N is a sequence length and the root is a prime number) (for example, refer to the Non-Patent document 2). If the sequence length: N=127, the correlation value is always kept 1/√{square root over ( )}127, and if the sequence length: N=128, the worst value (the maximal value) of the correlation value is 1/√{square root over ( )}2.
Sequences whose correlation value is 1/√{square root over ( )}N are abundant by the number of (N−1). From the viewpoint of a correlation value, it is proposed that the CAZAC sequence whose sequence length is the same primary number and whose parameter [parameter in the formula (1)] is different is allocated to each cell as a pilot sequence. As a result of that allocation, the number of sequences are (N−1), thus, the same pilot sequence needs to be re-performed for each of the (N−1) cells. The (N−1) will be called as the number of repeating of the pilot sequences below.
On the other hand, if the pilot sequence is sent in a plurality of blocks (two SB #1, #2 in the frame format shown in FIG. 19) as in the frame format of an uplink wireless access considered in the wireless communication system of the next generation (see FIG. 19) and if a pilot sequence is allocated for each cell as mentioned above (that is to say, if sent pilot sequence is common in a plurality of pilot blocks in a frame and the pilot sequences used in SB #1, #2 of the frame format shown in FIG. 19 are the same), an interference pattern from the other cell are the same in each pilot block at the receiving side.
That causes a problem in that no effect of reduction of interference by other cells by combining (averaging) a plurality of pilot blocks can be obtained at the receiving side. This is because no effect of reduction of interference by other cells can be obtained; as the pilot sequences sent in a plurality of pilot blocks are the same, interference from the other cells are received in the same manner (interference pattern) in every pilot block to combine (average) them (see FIG. 21).
If a pilot sequence common in a plurality of pilot blocks in a frame is used in a conventional W-CDMA (Wideband-Code Division Multiple Access) and the like, a sequence which is a random sequence multiplied for a frame called a scrambling code is sent. Thus, a pattern of the pilot sequence to be sent differs for each pilot block so that the effect of reduction of interference by other cells can be obtained by combining (averaging) a plurality of pilot blocks at the receiving side.
In the abovementioned conventional uplink wireless accessing system, if a sequence such as the abovementioned CAZAC sequence is used as a pilot sequence, it is limited that a scrambling code cannot be applied to. This is because that a unique characteristic is lost [for example, CAZAC characteristic (a characteristic advantageous for receiving such as constant amplitude in a time and frequency regions, and the cyclical self-correlation value is always 0 except for the case where the time shift is 0)], as a result of multiplying a sequence such as the CAZAC sequence by a random sequence such as a scrambling code.
If a sequence such as the CAZAC sequence is used as a pilot sequence and only a code is allocated for each cell, the problem in that the effect of reduction of interference cannot be obtained by combining (averaging) received pilot blocks in the abovementioned frame cannot be avoided.
The object of the present invention is to provide a wireless communication system, a pilot sequence allocation apparatus and a method for allocating pilot sequence to be used in the apparatus and a mobile station using the method that eliminates the abovementioned problems and can obtain the effect of reduction of interference by combining received pilot blocks if a sequence such as the CAZAC sequence is used as a pilot sequence.